In recent years, there has been a sharp rise in demand for portable compact electronic equipment, such as a small-sized headphone stereo, combination camera/video tape recorder and mobile communication terminal. These portable compact electronic equipment are equipped with rechargeable and high capacity secondary batteries as their power source and are charged by using a prescribed charger.
One type of the charger device is a contact type. This contact type charger has a spring-loaded point of contact. By contacting a point of contact of the portable compact electronic equipment side to the point of contact to electrically connect them, and then the charging current is supplied to the secondary battery built in the portable compact electronic equipment to form the electric path.
However, in these chargers, the point of contact may be oxidized or contaminants may be build up on the point of contact with time. These oxidation and contaminants cause the disconnection on the part of these contact points and hinder the supply of charging current to the secondary battery.
To avoid such problems, a charger using the noncontact type charging system has been considered. As the noncontact type charging system, a system of supplying the charging current from the charger to the secondary battery by the electromagnetic induction can be considered.
More specifically, a primary coil is provided on a terminal of the charger side and a secondary coil is provided on a terminal of the portable compact electronic equipment, and the primary coil and the secondary coil are brought close. When an electric current is fed to the primary coil under this condition, the primary coil generates the magnetic flux. At this point, if the current to be fed to the primary coil is turned ON or OFF at established intervals, the magnetic flux to be generated by the current induction varies with time. In the secondary coil side, an inducted electromotive force is generated by the electromagnetic induction due to the linkage of the magnetic flux which varies with time. The primary coil generates alternating current of which the direction of current is reversed according to the ON/OFF of the primary coil side as inducted current with the inducted electromotive force as a power supply. Thus, the noncontact type charger conducts charging by supplying the inducted current to be generated in the secondary coil to the secondary battery as the charging current.
With this arrangement, by bringing the primary coil of the charger side and the secondary coil of the portable compact electronic equipment side close, the electric power will be transmitted from the primary coil side to the secondary coil side using the magnetic contact by the electromagnetic induction, and thereby the noncontact type charger can be obtained.
In the charger constructed as described above, the primary coil and the secondary coil are built in the charger side and the electronic equipment side respectively, and by transmitting the power by the electromagnetic induction from the primary coil to the secondary coil, the noncontact power transmission can be conducted.
However, in this case, as the distance between the primary coil and the secondary coil is lengthened (in the case of magnetic permeability of the air), a coupling coefficient between the primary coil and the secondary coil becomes worse and the amount of linkage of the magnetic flux generated in the primary coil to the secondary coil decreases. Therefor, in the electric power transmission device, it is difficult to increase the degree of coupling between the primary coil and the secondary coil such as the general transformer.
Accordingly, the electric power transmission device described above has a problem that the efficiency in the power transmission becomes low due to the power loss by weak coupling.